1. Field of the Invention
The present invention generally relates to a friction brake used in automotive vehicles, aircraft landing gears, railroad cars and similar vehicles and, more particularly, to friction linings or pads used in the friction brake.
Specifically, the present invention pertains to the friction brake of a type comprising at least one friction element, such as a friction lining or pad, and a counter-friction element such as a driven member adapted to be braked by and in contact with the friction element.
2. Description of the Prior Art
The friction brake of the type referred to above is well known in the art such as exemplified by a single- or double-block brake, an external shoe brake and an internal shoe brake. In the automobile industry of today, for example, the external shoe brake is generally represented by the disk brake and the internal shoe brake is represented by the drum brake. In any event, the friction brake now widely used generally comprises at least one friction element or pad supported for movement in a direction close towards and away from a counter-friction or driven member movable relative to the friction pad, the movement of the driven member being braked when the friction pad is brought in contact therewith to apply a torsional friction force to the driven member.
More specifically, in the case of the automotive disk brake, the driven member is constituted by a brake disk mounted on a wheel axle for rotation together therewith, and the friction pad is disposed on each side of the brake disk, one or both of the friction pads being supported by means of a respective brake shoe for linear movement in a direction generally at right angles to the plane of rotation of the brake disk and also in a direction close towards and away from the brake disk. This equally applies to the drum brake except that the drum brake makes use of a drum, instead of the brake disk in the disk brake, and of the friction pads adapted to move in respective opposite directions either close towards each other (in external type) or away from each other (in internal type) to apply torsional friction forces to the peripheral wall of the drum.
Whatever the specific construction of the friction brake is, the friction brake wherein both of the friction pad and the driven member are made of carbon has come to be used in practice. Because of the frictional contact occurring between the carbon material in the friction pad and the carbon material in the driven member, this type of prior art friction brake can provide a high frictional resistance even when a high speed sliding contact takes place between the friction pad and the driven member. The use of carbon for each of the friction pad and the driven member brings about additional advantages in that any possible variation in frictional resistance with change in temperature can be minimized and in that, since the carbon itself has a high resistance to elevated temperature and a high thermal conductivity, the friction brake can be used in the environment where generation of frictional heat is considerable. Accordingly, the prior art friction brake utilizing carbon for both of the friction pad and the driven member is considered superior to, and is effective to provide a higher braking effectiveness than, the other prior art friction brake utilizing metal for both of them.
Moreover, the use of carbon makes it possible to manufacture a relatively light-weight friction brake that is suited for use in aircraft landing gears in which, because of the presence of high speed and high load environments, much difficulty is involved in adopting the friction brake utilizing metal for the friction pad and the driven member.
However, the use of carbon has been found posing a problem in that, after a long period of braking at low speed, a lubricating film tends to be formed on the contact surface of each of the friction pad and the driven member, accompanied by detrimental reduction in coefficient of friction. Therefore, if the prior art friction brake is used after the repeated braking at low speed to halt the driven member being driven at moderate or high speed, no braking effectiveness would be substantially available because of the presence of the lubricating films.
In order to obviate this problem, attempts have been made to add either chopped carbon fibers or continuous carbon filament fibers to carbon, but have not satisfactorily succeeded in the removal of the problem.
Accordingly, the friction brake employing carbon for each of the friction element and the counter-friction element cannot be effectively utilized in an application in which a substantially stabilized braking performance is required over a wide range from a low speed operating condition to a high speed operating condition. This can also be evidenced by the results of experiment conducted in an attempt to substantially obviate the above discussed problem.
Table 1 below illustrates change in coefficient of friction with change in sliding speed, that is, speed of relative sliding between the brake disk and the friction pads, which was attained when the brake disk, 240 mm in size and 22 mm in thickness, made of carbon and driven in one direction at a predetermined speed, and the friction pads each made of carbon and applied to the brake disk at a pressing force of 10 kg/cm.sup.2 for 10 seconds. The relative sliding speed was measured at a point on the brake disk spaced 194 mm from the axis of rotation of the brake disk.
As can be understood from Table 1 below, while the contact surface of each of the friction and counter-friction elements in the friction brake is recommended to give a coefficient of friction within the range of 0.3 to 0.4, the coefficient of friction has decreased with reduction in sliding speed.
TABLE 1 ______________________________________ Sliding Speed (m/min.) Friction Coefficient ______________________________________ 0.36 0.16 3.61 0.18 36.11 0.16 180.55 0.36 361.10 0.32 577.76 0.31 ______________________________________
Table 2 below illustrates change in coefficient of friction with types of material used for the friction and counter-friction elements. The relative sliding speed between the brake disk and the friction pads at the time of measurement was 36.11 m/min.
As can be understood from Table 2 below, while a relatively high coefficient of friction has been exhibited by a combination of carbon with a ferrous material such as the carbon steel and the gray cast iron, a reduced coefficient of friction has been exhibited by a combination of the carbon with any one of carbon, alumina and silicon carbide.
TABLE 2 ______________________________________ Material of Material of Coefficient Friction Pads Disk Brake Friction ______________________________________ Carbon Carbon 0.16 " Alumina 0.11 " Silicon Carbide 0.15 " Gray Cast Iron 0.37 " Medium Carbon Steel* 0.37 " High Carbon Steel** 0.35 " Low Alloyed Steel 0.33 " Stainless Steel 0.33 " Incoloy 0.31 " Inconel 0.30 " Hastelloy 0.31 " Stellite 0.32 ______________________________________ *0.35% carbon. **0.60% carbon.
Observation on the contact surface of each of the friction and counter-friction elements tested has shown that the contact surface of the element made of the ferrous material has been roughened with frictional scratches, that is, marks of friction, left thereon as extending in a direction parallel to the direction in which the relative sliding took place. Also, it has shown that, although the combination of carbon with the ferrous material did not result in the formation of lubricating films on the contact surface of any one of the elements, the combination of carbon with the other material than the ferrous material resulted in the formation of lubricating films on the contact surface of both of the elements.
Summarizing the above, friction between the elements, one made of carbon and the other of the ferrous material results in the formation of frictional scratches with little chance of formation of lubricating films, and therefore, it appears that a high coefficient of friction could be obtained even at a low speed operating condition.
Table 3 below illustrates change in coefficient of friction with change in sliding speed, that is, speed of relative sliding between the friction and counter-friction elements, after the friction and counter-friction elements, one made of carbon and the other of the ferrous material, have been brought into sliding contact with each other. The test conditions are substantially identical with those described in connection with Table 1 except for the difference in material used for the disk brake and the friction pads.
TABLE 3 ______________________________________ Sliding Speed (m/min.) Friction Coefficient ______________________________________ 0.36 0.35 3.61 0.30 36.11 0.37 180.55 0.30 361.10 0.23 577.76 0.19 ______________________________________
As can be understood from Table 3 above, a relatively high coefficient of friction is exhibited at a low speed operating condition, but decreases at a high speed operating condition.